Traditionally, these sleeves comprise a cylindrical bore of circular cross-section. Upon constriction, the radial stresses distribute themselves around the cylindrical bore which, owing to the vault effect, does not impart them completely to all outer strands of the heterogeneous cable. This well-known effect prevents the collapse of an arched structure under radial stresses acting concurrently on it from all sides. Likewise, under the action of these stresses, the outer strands come into contact with one another and, on account of the camber effect, fail to transmit entirely the forces received by them to the strands of the steel core. As a result, on the one hand, the steel core is poorly gripped and, on the other hand, the outer strands, made from a material having an elongation coefficient greater than that of the material of the inner strands and subjected to stresses higher than those exerted on the inner strands, elongate much more than these inner strands and form, at the exit of the sleeve, an enlargement usually termed "birdcage".
In order to obtain a better retention of the core, it is necessary to have sleeves exerting different radial clamping stresses upon the core and upon the sheath and requiring a separation of the strands of the sheath to obtain direct access to the ccore. This is a time-consuming operation and prevents mounting of the sleeve on a cable under electric voltage.